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In recent years, membrane bioreactors (MBR) have been increasingly used for advanced wastewater treatment to achieve higher effluent quality. MBRs for hydrogen driven denitrification have become one of the very promising options for lowering nitrogen concentrations in effluents. One of the major obstacles in extensive use of MBRs for this kind of treatment is excess precipitation and attachment of minerals on the biofilm/floc surfaces which may promote an increase in aggregate size, density and lead with time to reduction of hydrogen and nutrients diffusion and denitrification efficiency.

The impact that metal precipitants and extracellular polymeric substances may have on the hydrogen-driven denitrifiers' efficiency in long SRT systems has been evaluated in this study. Additionally the impact of the pH control on the mineral content, flocs structure and denitrification efficiency as a possible means of controlling system performance was tested. Two tested biomasses characterized with high mineral content of 75% and 65%. Extracellular polymeric substances (EPS) content remained constant regardless of mineral content and was equal to 0.086 +/− 0.03 g EPS/g VSS. The flocs created in two tested systems characterized also with low measured protein content 0.0081+/−0.0013 g proteins/g VSS which probably induced creation of big and stable flocs with average floc size equal to over 20000 μm2. Protective environment of big flocs possibly diminished the impact of variations in pH on bacteria performance and caused limitation in substrate diffusion. The determined denitrification rates remained stable regardless changes in reactors operating pH when no pH control was applied. The autotrophic denitrification rates obtained in this study were equal to 0.63+/−0.24 (pH=7–8) and 0.73+/−0.51 (pH>8.0) mg NO3-N/h*g VSS in reactor with 75% mineral. The results obtained for biomass with 65% mineral content were comparable and equal to 0.79 +/−0.50(pH=7–8) and 0.83+/−0.11(pH>8.0) mg NO3-N/h*g VSS, which is around 10 times lower then observations of other researchers. The nitrites accumulation was twice (pH=7.0–8.0) and five (pH>8) times higher in reactor with lower VSS/TSS ratio indicating possibility of inhibited diffusion of substrates in created biomass.

The application of carbon dioxide (CO2) and phosphoric acid as tool for pH control allowed to improve denitrification rates. The mineral content in both of reactors decreased to 71% and 60% when operational pH was maintained at 7.0 (+/− 0.15) via addition of gaseous carbon dioxide. The measured denitrification rate in both reactors increased to around 1.23+/−0.55 mg NO3−N/h*g VSS and 1.17+/−0.81 mg NO3−N/h*g VSS. The addition of phosphoric acid also allowed to increase denitrification rates up to 1.37+/−0.1 mg NO3−N/h* g VSS and 1.87+/−0.59 mg NO3–N/h* g VSS. The possible reason for improved denitrification was change in flocs morphology. Microscopic observation showed decrease in number of big flocs (>30000μm2) and increase by around 10% in the number of flocs with perimeter higher then 800 μm. This probably led to increase in active surface area and better substrate diffusion.
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Document Type: Research Article

Publication date: 2007-01-01

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